1. Field of Use
This invention relates generally to apparatus for controlling the capacity of a gas compressor such as is used in a refrigeration system or the like.
In particular, it relates to an improved unloader piston construction and arrangement in such apparatus whereby the unloader piston rings can be more easily replaced.
2. Description of the Prior Art
Reciprocating type gas compressors employ cylinders in which gas is compressed by reciprocably movable pistons. Typically, a suction or gas intake port and an independently operable discharge port are provided at the upper end of each cylinder. The suction port is provided with a suction valve having fully closed and fully open positions. In some reciprocating compressors unloading means are provided to "unload" the compressor, as during start-up or during operation when the full compressing capacity of the compressor is not needed. This can be accomplished by making one or more of the cylinders active or inactive, as by preventing full reclosure of the suction valve after each gas intake stroke and thereby enabling some or all of the gas entering the cylinder to exit through the intake port before it is compressed. In one type of compressor the cylinder takes the form of a hollow cylindrical sleeve or liner which is rigidly mounted on the compressor frame or housing. A removable cover, which is also part of the compressor housing, overlies the top end of the cylinder. The suction valve comprises an axially movable annular valve plate located at upper end of the cylinder directly beneath the cover. In compressor operation, the valve plate is repeatedly axially shiftably movable toward and away from the upper end of the cylinder between closed and open positions. Opening motion of the valve plate results from gas pressure differentials between the inside of the cylinder (low) and high gas pressure at the suction port which raises the valve plate during a suction stroke of the piston.
Capacity control means are provided to effect an appropriate shift of the valve plate to the positions of full closure or full open of the suction valve to thereby unload or load, respectively, that cylinder. One prior art form of capacity control means generally comprises an upwardly biased annular lift ring which is disposed around the cylinder sleeve and which is connected to the annular valve plate by lift pins which effect an upward shift and repositioning of the valve plate when the lift ring is raised. The lift ring is raised and lowered by a yoke assembly which engages the underside of the lift ring. The yoke assembly includes a yoke plate which is biased upwardly by yoke springs and is movable downwardly by a fulcrum pin which is welded to the yoke plate and pivotally connected intermediate the ends of a pivotally mounted yoke lifting arm or lever. One end of the lifting arm is pivotally connected to a base plate which has a fixed location relative to the compressor frame. The other end of the lifting arm is pivotally connected by a removable pivot pin to the lower end of an unloader piston rod which has its other end connected to an unloader piston. The unloader piston is located in and axially movable in an unloader cylinder formed in the compressor frame directly beneath the aforementioned removable cover. The unloader piston is movable axially to actuate the yoke lifting arm in response to gas (or oil) pressure in the unloader cylinder; the gas (or oil) pressure being indicative or representative of system conditions requiring loading or unloading. The unloader piston has one or more piston rings which need to be periodically replaced. Heretofore, this was accomplished by removing the compressor housing cover, the safety head spring, the safety head yoke, the safety head, the cylinder, the piston and its connection rod and then removing a hand-hole cover plate on the side of the compressor housing to gain access to the unloader piston rod and its pin and to gain access to yoke assembly within the compressor housing. Then, it was necessary to remove the pivot pin which connects the lifting arm to the unloader piston rod, withdraw the unloader piston and its attached unloader piston rod from the unloader cylinder, replace the piston rings, and re-assemble the apparatus. Since the pivot pin which connects the lifting arm to the piston rod is relatively inaccessible and difficult to remove and re-install, piston ring replacement is a difficult, time-consuming and costly procedure.